Calculate the percentage of phosphorus in:

(a) Calcium hydrogen phosphate, Ca(H₂PO₄)₂

(b) Calcium phosphate, Ca₃(PO₄)₂

(a) Relative molecular mass of Ca(H₂PO₄)₂
     = 40 + 2(2 x 1 + 31 + 4 x 16)
     = 40 + 2(2 + 31 + 64)
     = 40 + 194
     = 234 amu

Wt. of P in Ca(H₂PO₄)₂ = 2 x 31 = 62 g

% of P = $\dfrac{\text{Wt. of P}}{\text{Total Wt. of Ca(H}$2\text{PO}4)_2}\times 100

= $\dfrac{62}{234}$ x 100 = 26.5%

∴ Phosphorus in Calcium hydrogen phosphate is 26.5%

(b) Relative molecular mass of Calcium phosphate, Ca₃(PO₄)₂
     = 3 x 40 + 2(31 + 4 x 16)
     = 120 + 2(31 + 64)
     = 120 + 190
     = 310 amu

Wt. of P in Ca₃(PO₄)₂ = 2 x 31 = 62 g

% of P = $\dfrac{\text{Wt. of P}}{\text{Total Wt. of Ca}$3\text{(PO}4)_2}\times 100

= $\dfrac{62}{310}$ x 100 = 20%

∴ Phosphorus in Calcium phosphate is 20%